Add ability to create keys

[etopian]

The ability to create and update keys should be added to user. Something trivial to do.

https://developer.github.com/v3/users/keys/

[acrobat]

This is already implemented and possible through the currentuser -> keys api.

$client->user()->keys()->create(array('title' => 'key title', 'key' => 12345));

I've created a pull request to add the missing docs for the public keys api. See #478